Most magnetic recording media are based on acicular gamma-Fe.sub.2 O.sub.3 particles, by "acicular" being meant that the particles have an average length-to-width ratio of more than 2. Typically the H.sub.c of such media may be about 300-350 Oersteds and possibly as high as about 400 Oersteds. It is known that by modifying such particles with cobalt ions in the form of cobalt oxide, the H.sub.c can be increased to substantially higher levels. Such increased H.sub.c enables improved storage of high frequency information.
While acicular gamma-Fe.sub.2 O.sub.3 particles derive their magnetic anisotropy primarily from shape, modification with cobalt oxide introduces considerable crystalline anisotropy. As a result, the easy axis of magnetization tends to deviate from the physical or long axis. When heated and/or subjected to mechanical stress, and easy axis which deviates from the physical axis tends to shift its orientation or direction. Thus near-neighbor particles tend to interact magnetically and cause an irreversible decrease in the recorded magnetization. When the recording media are in tape form, the shift in orientation tends to result in undesirable layer-to-layer print-through characteristics. [See Flanders, P. J., "Changes in Recording Tape Magnetization Produced by Stress", IEEE Transactions on Magnetics, Vol. MAG-12, No. 4, page 348, July 1976.] The cobalt-containing acicular gamma-Fe.sub.2 O.sub.3 particles of U.S. Pat. No. 3,117,933 (Abeck), had they been used commercially, would have been highly subject to the aforementioned instabilities. Considerable improvement was achieved in U.S. Pat. No. 3,725,126 (Haller et al.)
The recent trend toward thinner tape backings and recording at shorter wavelengths has created a need for better thermal and magneto-mechanical stability than that provided by present commerical recording tape which employs cobalt-containing acicular gamma-Fe.sub.2 O.sub.3 particles. By converting some of the Fe.sup.+3 to Fe.sup.+2, the magneto-mechanical stability can be improved, but this may introduce another problem, i.e., significantly increasing H.sub.c during storage. In one case where 20% of the iron was Fe.sup.+2 and the cobalt content was 1.2%, a tape experienced an increase in H.sub.c from 512 Oersteds to 763 Oersteds during 20 months' storage at ordinary room temperature. After 30 minutes at 100.degree. C, the H.sub.c of the tape returned essentially to its original level and again began gradually increasing upon further storage at room temperature.
In recent attempts to eliminate the above-discussed problems, the cobalt has been applied to the surface of acicular iron oxide particles which in some cases have appreciable Fe.sup.+2 content. For example, see TDK's U.S. Pats. Nos. 3,958,068; 4,069,367; 3,977,985; 3,953,656 and 4,010,310 and German Offenlegungsschrift No. 2,410,517; Fuji Offenlegungsshrift No. 2,510,799; also Hitachi's Japanese patent application No. 143934/1974 (Publication No. J 51070498). None of these patents or patent applications discloses the preparation of the acicular iron oxide core. Typical of these are TDK's U.S. Pats. No. 3,977,985 wherein the iron oxide is magnetite and No. 4,010,310 wherein the iron oxide has an Fe.sup.+2 /Fe.sup.+' ratio of 0.1 to 0.35. In each example of both of those patents, an aqueous cobalt salt solution is added to an aqueous slurry of the particles and the cobalt is precipitated by means of added base as cobalt hydroxide onto the surfaces of the particles, followed by heating to dehydrate the hydroxide. At least two earlier steps were required to obtain the acicular iron oxide particles, namely, reduction and oxidation of a ferric oxide hydrate.
TDK has recently introduced into the U.S. a tape labelled "Avilyn" which appears to employ acicular iron oxide particles having a cobalt compound at the surface; IEEE Transactions on Magnetics, Vol. MAG-10, No. 3, page 655, September 1974. The "Avilyn" tape largely avoids the above-discussed problems. It is not known whether it was made by a process disclosed in any of the above-listed TDK German patent applications.